Single-Layer = servers function as compute nodes as well as storage nodes.
Dual-Layer = servers function only as storage nodes; compute runs on different nodes.

In a Single-Layer architecture, the servers with Virtual SAN software function as compute nodes as well as storage node; StarWind performs storage synchronous replication

In a Dual-Layer architecture, StarWind Virtual SAN software replicates data in active-active mode between the dedicated storage servers and provides HA storage for use by separate compute nodes that do not have StarWind Virtual SAN installed.